Still bothered by the events on Traken, the Doctor decides to make his
TARDIS work like the Master's did and repair it's
chameleon circuit, the device that should allow it to change it's outer
shape. What the Doctor fails to realize, however, is that
his plans have been anticipated by a newly regenerated Master, who secrets his
TARDIS within the Doctor's so that he will be taken along to the alien city of Logopolis.
The Doctor hopes that the Logopolitan's unique form of mathematics will restore
his chameleon circuit, but the Master's interference with the city's workings
doom not only the Doctor, but the entire universe as
well...

This story sees the departure of Tom Baker as Doctor Who, and the debut of
Janet Fielding as Tegan and Anthony Ainley
as the Master.

starring Tom Baker as Doctor Who, Matthew Waterhouse as Adric, Sarah Sutton
as Nyssa, Janet Fielding as Tegan, and Anthony Ainley as the Master. Written by Christopher H. Bidmead, Executive Produced by Barry
Letts, Produced by John Nathan-Turner, and Directed by Peter Grimwade.

Originally transmitted from 28 February 1981 to 21 March 1981 on BBC1.